Finger print sensors comprising electrodes for measuring characteristics in a finger surface are well known. For example, EP0988614, U.S. Pat. Nos. 5,963,679, 6,069,970 and 8,421,890 describe sensors based on different impedance or capacitance measurement principles with strip-shaped or matrix sensors comprising a number of individual sensor elements.
It is in the nature of a fingerprint sensor that a finger will have to contact, or be in close proximity to, the fingerprint sensor for the sensor to be able to sense a fingerprint pattern. Ideally, the finger is considered to be grounded through the body. As a practical matter, however, the body functions like an antenna that picks up radiation or electronic noise from the surroundings. The electronic noise propagates through the body to the finger and the fingerprint sensor and interferes with signals being detected by the sensor. This human body interfering signal often has the same order of magnitude as the signal difference between a ridge and a valley that are being measured. In addition to radiated noise, power supply noise can be conducted into the sensor through the capacitive connection of the human body to a different ground potential than the sensor.
In addition to noise through the human body, sensor-to-sensor and intrasensor variations may arise due to differences in individual grid crossover junction etching, dielectric variations, and coating variations.
Current art fingerprint sensors typically attempt to reduce these interfering system noises through post detection filtering methods rather than employing advanced electronic sensing methods designed to mitigate these noise sources. There is therefore a need in the industry for an improved fingerprint sensor with that is specifically architected to improve the rejection of the typical conducted and radiated noise sources found in fingerprint sensors.